IMPLEMENTATION OF LOW COST SWITCHED RELUCTANCE MOTOR …€¦ · Switched Reluctance Motor is a...

JIGNESH MAKWANA* et al. ISSN: 2250–3676

[IJESAT] INTERNATIONAL JOURNAL OF ENGINEERING SCIENCE & ADVANCED TECHNOLOGY Volume-2, Issue-4, 772 – 780

IJESAT | Jul-Aug 2012

Available online @ http://www.ijesat.org 772

IMPLEMENTATION OF LOW COST SWITCHED RELUCTANCE

MOTOR DRIVE USING RT-LAB

Jignesh Makwana1, Ambarisha Mishra

2, Pramod agarwal

3, S.P Srivastava

4

1Research Scholar, Electrical Department IIT Roorkee, Uttrakhand, India, [email protected]

2Research Scholar, Electrical Department IIT Roorkee, Uttrakhand, India,[email protected]

3Professor & Head, Electrical Department, IIT Roorkee, Uttrakhand, India, [email protected]

4Professor, Electrical Department, IIT Roorkee, Uttrakhand, India, [email protected]

Abstract This paper demonstrates the implementation of low cost switched reluctance motor (SRM) drive and application of RT lab as real time

hardware-in-loop (HIL) controller. Split DC converter and positioning sensing arrangement is developed for 500W 8/6 pole SRM.

Control part is implemented using Opal RT Lab technology. Application and optimistic characteristics of proposed low cost drive are

described. Experimental results of performance and efficiency for proposed SRM drive are presented which shows very low cost

versus performance ratio compare to induction motor and permanent magnet motor drive.

Index Term— Converter, reluctance motor, RT Lab, electric drives

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1. INTRODUCTION

The Switched Reluctance Motor (SRM) drive promises an

impressive set of benefits over its competition includes high

efficiency over a wide speed range and partial loads, high-

speed capability, easy cooling with heat source only in the

stator, ruggedness for high-temperature or vibration

environments, and relatively simple mechanical construction.

But sheer numbers of induction and brushless permanent

magnet (PM) motors at work in industrial and commercial

applications testify to their well-established manufacturing

infrastructure and user acceptance. This has limited wide use of

SRM - a technology that offers a practical alternative for

various demanding applications. Perhaps today‟s growing

demand for energy efficiency motivates the users and

companies to look at SRM as an alternative comes from

concern about magnet material cost in PM synchronous motors

and a desire to move away from induction motors for overall

efficiency and system cost.

Current resurgence in demand is observed for SRM drive with

a variety of platforms intended for industry, includes high

speed applications such as screw compressors, blowers, and

high-speed pumps and low-speed, high-torque areas

(conveyors, feeders).

SRMs can‟t run direct-on-line, thus require an associated power

converter (drive) to complete an SRM drive system. SRM

power converter topology differs from that of conventional ac

drives in the arrangement of power switch and fly-back diode

circuits. For smaller drives, use of power modules is a cost-

effective design route, but off-the-shelf modules are not

available for SRM as for other motor technologies. As more

applications become variable speed, the SRM option, whose

cost is competitive with an equivalent inverter-fed induction

motor, becomes viable across a growing range of applications.

This paper presents the development of low cost SRM drive

which includes development of split DC controller and open

loop controller with position sensing arrangement. Fixed

frequency PWM controller is developed and implemented using

Opal RT Lab.

A single phase induction motor achieved worldwide acceptance

for general purpose motor drive in domestics and industrial

application because its feature to run direct on AC lines without

having costly converters. Beside large number of converter and

control modules are readily available today for induction motor

and brushless DC motors. While there are no such a converter

and controller modules are available for the SRM which

discourage the usage of the SRM technology which offers a

high performance and efficiency. Low cost SRM drive

presented in this paper is to show the performance of SRM to

run directly on AC mains supply with low cost but reliable

converter and position sensing arrangement without starting

hesitation.





2. SWITCHED RELUCTANCE MOTOR

Switched Reluctance Motor is a doubly salient and singly

excited motor. Unlike conventional AC or DC motor which

required either two winding or one winding and one permanent

magnet to produce the torque SRM have only winding in the

stator. The rotor has no windings, magnets or cage windings

but is built up from a stack of salient pole laminations. Torque

is produced due to force of attraction between magnetic field of

stator winding and magnetic material of rotor. SR machines

offer a wide variety of aspect ratios and salient pole topologies.

Each application is likely to a better suited to a specific SR

topology. Fig. 1 shows the geometry of four phase SRM having

8 stator pole and 6 rotor pole which denoted by 8/6 SRM in

general. Generally, selection of higher number of phase and

pole reduces the torque ripple, but it required more switching

devices. Some important old references of the SRM are [1]-[5].

Fig-1: Geometry of four phase 8/6 SRM

When current is passed through the phase winding the rotor tends

to align with the stator poles and it produces a torque that tends

to move the rotor to a minimum reluctance position. The

direction of torque generated is a function of rotor position with

respect to energized phase, and is independent of direction of

current flow through phase winding. Continues torque can be

produced by intelligently synchronizing each phase‟s excitation

with the rotor position. An equivalent expression of torque is,

constanti

cwT

(1)

or

constant

f

i

wT

(2)

Where cw and fw are co-energy and stored field energy

respectively. Mathematically,

diwc (3)

and diw f (4)

Fig. 2 shows the typical magnetic characteristics of the SRM

which represent the number of magnetic curves relates flux

linkage and phase current for unaligned to aligned position of

rotor. Its shows the two saturation mainly due to pole corner

saturation near unaligned position at lower current and due to

saturation of yoke near aligned position at higher current. If

magnetic saturation is neglected then the relation between flux-

linkage and current at an instantaneous position θ is a straight

line whose slope represents an instantaneous inductance L.

Thus Ψ = Li and,

2

2

1Liww fc (5)

Therefore torque d

dLiT 2

2

1 N-m (6)

Fig.-2: Typical flux linkage characteristics of SRM

Variation of idealized phase inductance is shown in Fig. 3. To

develop continuous torque in positive direction it is required to

energize the phase only during their respective rising inductance

period as shown in Fig. 3 which explains the necessity of

position sensor to command the phase current.

Different converter topology may be use to energize the phase of

the SRM but most common is two switched per phase

asymmetric converter shown in Fig. 4. There are number of

converter topology is published in the literature to reduce the

number of switches per phase and reduce the cost of converter

and firing circuit [6]-[11].





Fig-3: Inductance profile of four phase SRM

Fig-4: Asymmetric bridge converter

There are several methods to control the torque-speed and the

position of the SRM. Hysteresis current control and PWM

control are two low cost and simplest methods for easy

implementation. In hysteresis control phase switch turned off and

on according whether the current flowing through the winding is

greater or less than the reference current, while in PWM control

fixed frequency variable duty cycle scheme can be employed to

regulate the current as shown in Fig 5.

3. POSITION SENSING ARRANGEMENT AND

PWM CONTROL STRATEGY

There are so many options for choosing position sensing

scheme for the SRM drive including absolute or incremental

encoder, Hall Effect sensors or even many sensorless methods

have been developed [12]-[16]. Low cost high speed position

sensing arrangement with control scheme is shows in Fig 6.

Toothed disk having teeth symmetrical to the rotor pole is

attached on shaft and is in perfect synchronization with the

rotor pole. Disk cuts the light emitted by the source which

generates two digital pulses to decide commutation period of

the phase. By combining high speed TTL logics individual

commutation pulse can be generated for all phases which mixed

with the PWM signal to achieve current control as shown in Fig

6. For easy and flexibility Opal RT Lab is used to implement

controller part.

Fig-5: PWM and hysteresis current control

Fig. 7 shows the commutation pulse C4 and C3 which decide the

conduction period of phase 4 and phase 3 respectively while

conduction period of phase 2 and phase 1 is decided by

commutation pulse C2 and C1 which are logically invert of C4

and C3 respectively. Fig. 8 shows that commutation pulse is

logically mixed with the fixed frequency PWM pulse and

generated gate pulse are applied to the isolated MOSFET driver

circuit shown in Fig. 9 to achieve the current and speed control of

the SRM.

Fig-6: Position sensing arrangement and control logic





Fig-7: Commutation pulse to decide on-off instant of phase

Fig-8: Gate pulse; Commutation pulse combined with PWM pulse

4. SPLIT DC CONVERTER

Unlike conventional AC and DC motor SRM cannot run with

direct AC or DC supply. SRM require converter circuit to guide

the current in appropriate phase with rotor position sensing

arrangement. Fig. 9 shows split DC one switched per phase

converter circuit for SRM. Simple diode bride with filter is

added to AC to DC conversion. Here main aim is to obtain

performance characteristics of 0.5KW SRM with 230 V AC

mains. Assume that there is no magnetic saturation that means

inductance is unaffected by the current. Also neglecting the

mutual inductance for the simplicity voltage equation of the one

phase is,

d

diR

dt

diRV

phmphph

phphphph (7)

Where phV is the phase voltage equal to 2dcV and

rmsdc VV 2

d

dLi

dt

diLiRV

phphm

phphphphph

)()( (8)

One switched and one diode is associated with each phases. At

any instant two phase are ON to maximize the torque and

which also minimize the torque ripple. Alternative phases (1,3

and 2,4) are never going to conduct simultaneously. It also

helps in balancing the capacitor C1 and C2. Fig. 10 shows the

mode of operation of the converter. Mode 1 is phase energize

mode and mode 2 is regenerating mode. When M1 is ON

voltage across phase is Vdc/2 and current is circulating through

C1, M1 and Phase 1. At the instant of turning OFF M1; diode

D1 comes in conduction and current circulate through the D1,

Phase 1 and C2.

Fig-9: One switched per phase split DC converter with AC

mains

Mode 1 Mode 2

Fig-10: Mode of operation of split DC converter

5. OPAL RT-LAB TECHNOLOGY

Real-time simulation of SRM drives on a CPU-based real-time

simulator can produce accurate results, but can also have the

undesirable effect of causing current overshoots because of

model latency. To remedy this problem, an FPGA

implementation is desirable because it offers a very low

calculation time and I/O latency.





RT-LAB, from Opal-RT Technologies, is a real-time

simulation platform that enables real time and HIL (hardware in

loop) simulation of controllers, electric plants or both, through

automatic code generation methods. The entire process occurs

without the need for handwritten „C‟ code, enabling very rapid

deployment of prototyped controllers or HIL-simulated plants.

The process is notably very efficient when applied to I/O code

because RT-LAB provides a set of simulink blocks that

automatically configure common I/O functions, like analog

input/outputs and time-stamping capable digital I/Os, with a 10

nanosecond resolution. Special interpolating models use this

timing information to greatly increase simulation accuracy [17].

RT-LAB simulator is equipped with a user-programmable

FPGA card. The FPGA card can be programmed with the

Xilinx system generator blockset for simulink enabling

implementation of complex sensor models like resolvers,

Resolver-To-Digital and FM resolvers or even complex motor

drives [18], [19].

RT-Lab is used as real time hardware-in-loop controller in this

implementation for easy and flexibility.

Table I summarizes the characteristics of FPGA board used in

this paper and Table II summarize the input output card used

for analog output of phase current and gate pulse.

Table-1: Reconfigurable FPGA Boards

Table-2: Input output configuration

6. HARDWARE IN LOOP CONTROLLER

As shown in Fig. 6 two rotor position signals are applied to the

analog input card of RT- Lab. Here logic operation is

performed to mix the fixed frequency PWM control signal to

control the current and speed of the motor. From analog output

card four gate pulses are taken out and supplied to the

MOSFET driver circuit as shown in Fig. 6. Duty cycle of the

PWM pulse can be controlled in real time to control the motor

speed. User interface is provided to control and record/observe

the motor speed in real time.

RT Lab allows to model a subsystem in MATLAB simulink

environment with some own rules and perform automatic code

generation and transfer of the simulink model for the FPGA

implementation. Fig. 11 shows the subsystem modelled for the

present controller. Subsystem named “SM_speed_control”

contain the model of the actual controller while subsystem

named “SC_speed_control” represent the model for user

interface for online parameter control and monitor. Fig. 12

shows the modelling of controller and Fig. 13 shows user

interface panel available for HIL speed control and monitor.

Fig-11: Controller subsystem

Fig-12: Subsystem model of controller

Fig-13: User interface available for real time control and

monitor





7. PERFORMANCE CHARACTERISTICS OF SRM

DRIVE

Fig. 14 to Fig. 22 shows the different performance plots for the

projected SRM drive include speed-torque characteristics,

efficiency, power-factor, no-load input power, noise analysis

and vibration details. Fig.23 shows the commutation pulse with

the phase current at no-load speed of 1100 rpm. Fig. 24 shows

the phase voltage and phase current waveform for the motor

speed of 880 rpm and load torque of 4 kg-cm. Fig. 25 shows the

experimental setup for the proposed SRM drive.

Fig-14: Speed torque characteristics

Fig-15: Efficiency versus speed

Fig-16: Efficiency versus load

Fig-17: Steady state speed versus PWM duty cycle

Fig-18: No-load current versus speed

Fig-19: No load power input versus speed

Fig-20: No load power factor versus speed





Fig-21: Noise performance versus speed

Fig-22: Vibration performance versus speed

Fig-23: Commutation pulse and phase current at no load speed

of 1100 rpm

Fig-24: Phase voltage and current at 880 rpm and load of 4 Kg-cm

Fig. 25 Experimental setup

8. CONCLUSION

Projected scheme shows impressive advantages over

conventional motor drive regard in motor, converter and control

electronics. Motor offer maintenance free robust performance

with low manufacturing cost and low material cost. Rotor

inertia is very low because of salient pole type construction

lead to low weight and small size compare to conventional AC

and DC motor. Stator is simple to wind; end turns are short and

robust and have no phase-phase crossovers support low cast

easy manufacturing steps and also easy to repair. In most

applications the bulk of the losses appear on the stator which is

relatively easier to cool. Because there is no any costly

permanent magnet on rotor permissible rotor temperature is

high compare to permanent magnet motor in cost effective way.

Motor provide higher torque compare to commutator motor and

induction motor at all speed. Furthermore starting torque can be

very high without the problem of excessive inrush currents and

extremely high speed is possible. Motor is fully resistant to

environment contras to the permanent magnet motor.

SRM Controllers add to the benefits, since they do not need a

bipolar (reversed) device because torque is independent of

direction of current. One switched per phase MOSFET

controller is cost effective compare to inverter particularly for

brushless permanent magnet machines. It offers fault tolerant

operation with one or more faulty phase or even with shorted

MOSFET. It is found experimentally that with one phase open

motor is running with 80% of its full capacity and with one

MOSFET shorted motor is running with less efficiency and





capacity because one phase is always remains on irrelative of

rotor position which generate negative torque and required

more starting current. In addition projected converter allows

two-phase excitation at a time which reduces the ripple in the

torque.

Furthermore digital controller and MOSFET driver add to the

benefit of low cost in simplest way. MOSFET driver circuit

required only three isolated power supply while mostly used

asymmetric bridge converter of SRM requires five. Low

frequency PWM speed controller offers benefits over hysteresis

controller that it does not required a single current sensor while

hysteresis controller requires four current sensors for reference

current and four individual controllers. Low frequency PWM

control reduces the switching losses and acoustic noise thus

increases the performance and efficiency in simple and cost

effective way. Body mounted infrared positioning scheme add

the benefits of cost with compare to costly encoders with

reliable performance with CMOS and TTL logics for very high

speed performance.

Result dictates that the cost versus performance ratio of the

proposed SRM drive is quite low. It‟s observed that proposed

SRM drive gives rugged performance with 230V AC mains

supply without starting hesitation.

Use of RT-Lab is much time saving in developing a control

model for the practical electric motor drives and offer great

easy and flexibility.

Counter part of the proposed drives is the level of acoustic

noise production which prevents the use of SRM for the

domestic application like fan and other continuous duty

application.

APPENDIX

Motor Specifications:

Duty Type continuous

Motor Type 8/6 four phase SRM

Output power 0.5 KW

Phase voltage 150 V

Number of turn per phase 310 turns per phase,

Resistance per phase 4.5 ohm per phase,

Stator outer diameter 90.8mm

Rotor outer diameter 48.4mm

Electronics specifications:

Power switch: IRFP450A

Diode: MUR1560

PWM frequency 1.66 KHz

ACKNOWLEDGMENT

Author is thankful to the electric department of Indian Institute

of Technology Roorkee for providing required equipments for

experimental setup.

REFERENCES

[1] Lawrenson, P.J., and AGU, L. A.: „Theory and

performance of polyphase reluctance machines‟, IEE Proc.

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pp. 253-265

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127

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469–476, 1990.





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algorithm for an SR motor drive system without position

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BIOGRAPHIES

Jignesh Makwana received the B.E and

M.E degrees in Electrical Engineering from

the Birla Vishvakarma Mahavidhyalaya,

v.v.nagar, gujarat, India, and L.D.

Engineering College, ahemadabad, gujarat,

India in 2004 and 2006 respectively. He was

a lecturer with the C.U. Shah College of

Engineering and Technology from 2006 to

2008 and joined the R.K College of Engineering and

Technology in 2008. Currently he is a research scholar in

Electrical Department of Indian Institute of Technology,

Roorkee, India. His fields of interest are electric machines,

drives and power electronics.

Ambarisha Mishra was born in 1986. He

received B.Tech. (Electrical) from Uttar

Pradesh.Technical University Lucknow, India,

in 2007 and M.Tech.(Power Electronics &

Drives) from National Institute of Technology

Kurukshetra, India, in 2009. Currently he is

pursuing PhD from in Electrical Engineering Department,

Indian Institute of Technology Roorkee, India. His field of

interest includes electric drives and power electronics.

Pramod Agarwal received the B.E., M.E.,

and Ph.D degrees in Electrical Engineering

from the University of Roorkee, India, in

1983, 1985 and 1995, respectively. He joined

the erstwhile University of Roorkee, India in

1985 as Lecturer. He was a Postdoctoral

Fellow with the Ecole de technologie superior, University of

Quebec, Montreal, Canada. He is currently a Professor with the

Department of Electrical Engineering, Indian Institute of

Technology, Roorkee, India. He has developed a number of

educational units for laboratory experimentation. His fields of

specialization are electrical machines, power electronics,

microprocessor and microcomputer controlled ac/dc drives,

active power filters, multi-level inverters and high power factor

converters.

S. P. Srivastava received the bachelor's

and master's degrees in Electrical

Technology from I.T. Banarus Hindu

University, Varanasi, India in 1976, 1979

respectively and the Ph. D degree in

Electrical Engineering from the University

of Roorkee, India in 1983. Currently he is

with Indian Institute of Technology (IIT) Roorkee, India, where

he is a Professor in the Department of Electrical Engineering.

His research interests include power apparatus and electric

drives.

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IMPLEMENTATION OF LOW COST SWITCHED RELUCTANCE MOTOR …€¦ · Switched Reluctance Motor is a...

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